The present invention generally relates to methods and apparatus suitable for stabilizing and reducing the effect of noise in a sensing operation of magnetoresistive random access memory (MRAM). More particularly, the invention provides methods and apparatus for stabilizing and reducing the effect of noise in a sensing operation of MRAM caused by capacitive interconnects between elements changing states during MRAM operation.
Typical MRAM structures have a nonmagnetic layer sandwiched between two ferromagnetic films. MRAM employs the magnetoresistive properties of this structure to store data. In each storage element, an MRAM employs two orthogonal lines, commonly termed a word line and a sense string, in order to detect the magnetization direction of these magnetic thin films. Each sense string includes a magnetic thin film that serves as a memory element, and each word line generally addresses multiple sense strings. Magnetic thin films set to have a parallel moment have a low resistance and are typically assigned the xe2x80x9conexe2x80x9d state. Magnetic thin films set to have an anti-parallel moment have a high resistance and are typically assigned the xe2x80x9czeroxe2x80x9d state, but may also be assigned to the xe2x80x9conexe2x80x9d state.
During a read operation, a word current passes through the word line generating a magnetic field, thereby changing the resistance of the magnetoresistive material in the sense string. A sense current passes through the sense string. A sense line receives the signal from the sense string. A differential amplifier compares the signal from the sense line to a reference line to determine whether a one resistance or a zero resistance is stored in the MRAM. A differential amplifier notes the change in voltage across the sense line to determine the resistive state of a storage element.
Because the change in the voltage across the sense line is relatively small, on the order of 2-12 mV, noise is a significant factor affecting both the speed and the accuracy of the read operation. One primary source of noise is due to capacitive coupling from the word line to the sense strings during the rapid voltage change of the word line. The noise levels resulting from the capacitive coupling may typically exceed signal levels by a factor of ten or more. Furthermore, capacitive coupling varies in magnitude from one sensing operation to the next.
To compensate for the noise factor, various strategies may be employed. The sensing operation may be adjusted to allow for integration of the sensing signal over a longer time period. These strategies, however, have the effect of reducing the performance and efficiency of MRAM. Longer sensing operations reduce the operational speed of MRAM. Other strategies, such as using the magnetic tunneling junction, may be employed to obtain a larger signal. These methods do not address the problem of injected noise. Each sense string along the active word line provides additional capacitive coupling, and activation of the word line injects noise into each sense string. Thus, as MRAM arrays increase in size, the noise from capacitive coupling increases proportionally.
Stabilizing and reducing the effect of noise due to the activation of the word line detected by the differential amplifier would provide more accurate resolution of the read operation and quicker resolution of the sensing signal, allowing for faster and more reliable operation of MRAM. Increasing the sensing signal, while reducing and stabilizing noise, would further improve resolution of the sensing signal.
Use of multiple memory spots in each bit can increase the sensing signal, thus improving the signal to noise ratio. A number of factors limit the signal that can be obtained using a single memory element. Space considerations limit the width and length of each individual memory element. One constraint on the length of a memory element is the width of the word line. The word line must carry a current that generates a magnetic field sufficient to address the entire bit. Taking into account manufacturing variation, the memory element must be short enough to allow for remaining within a consistent field supplied by the word line. Thus, having too great a length may result in an individual memory element having inconsistent switching characteristics. Furthermore, inconsistent switching characteristics result in loss of uniformity through a memory array. Multiple memory spots, connected in series would increase the strength of the sensing signal in proportion to the number of memory spots while maintaining consistent switching characteristics.
Use of multiple memory spots in each bit can also improve reliability of MRAM memory arrays. If manufacturing defects result in one memory element of a multiple memory element bit being unreliable, the remaining memory spots can enable the bit to remain functional. This redundancy improves yield and lowers cost of manufacturing MRAM chips. Likewise, if electromigration or other operational hazard causes a memory element of a multiple memory element bit to malfunction, the remaining memory spots provide for continued usability of the MRAM chip. This provides for improved reliability of the MRAM chip.
Thus, there is a need for methods and apparatus for stabilizing and reducing noise during a sensing operation of MRAM that do not suffer from the deficiencies of prior apparatus. These methods and apparatus should ensure accurate read operations without necessitating longer read operations.
There is a further need for methods and apparatus for stabilizing and reducing noise allowing for larger MRAM applications.
There is a further need for methods and apparatus for stabilizing and reducing noise allowing for faster MRAM operation.
There is a further need for methods and apparatus that provide a greater differential in the signal from the sense circuit.
There is a further need for methods and apparatus for increasing the magnitude of the sensing signal.
There is a further need for methods and apparatus that allow for greater memory capacity of an MRAM array.
The present invention solves these needs and other problems in the field of MRAM sensing apparatus and methods by providing, in most preferred aspects, apparatus and methods for stabilizing and reducing noise present during MRAM sensing operations. The apparatus couples sense strings intersecting a word line to both a sense line and a reference line. Coupling of a portion of the sense strings to the sense line and a portion of the sense strings to the reference line reduces the amount of noise injected into the sense line. The coupling also balances noise between the sense line and the reference line in order to stabilize and reduce noise detected by the sensing apparatus.
It is therefore an object of the present invention to provide novel methods and apparatus to stabilize noise during a sensing operation of MRAM.
It is another object of the present invention to provide such novel methods and apparatus to reduce noise during a read operation of MRAM.
It is a further object of the present invention to provide such novel methods and apparatus to increase the operational speed of an MRAM read operation.
It is a further object of the present invention to provide such novel methods and apparatus to increase the magnitude of the difference in the sensing signal.
It is a further object of the present invention to provide such methods and apparatus to allow for greater capacity of MRAM arrays.
These and further objects and advantages of the present invention will become clearer in light of the following detailed description of an illustrative embodiment of this invention described in connection with the drawings.